Humans experience a life cycle similar to other mammals, starting with an embryo and ending at death. A life cycle begins at conception. The average life of a human is about 67 years.
After an embryo has developed into a fetus, it readies itself for birth. It is then born into the world as an infant. This stage lasts for three years. The most development occurs in this first part of the life cycle.
From ages three to twelve, humans experience the stage of childhood. For these nine years, humans develop psychologically and physically.
The teenage years, also known as adolescence, are those in which humans develop sexually. This is when sex organs mature, and hair grows on more parts of the body as a result of hormone development.
Adulthood is usually the longest lasting stage in the human life cycle, stretching from age 20 to 50. By age 35, adults are considered "middle age" until they become mature adults.
From ages 50 onward, mature adulthood finishes off the life cycle. If humans live beyond age 75, they have entered into "late adulthood". Death is the end of the life cycle.